Slowly, in fits and starts, Fox’s “X-Men” became the franchise nobody wanted — and the latest, and possibly last, chapter in the saga, “Dark Phoenix,” is the sad, misshapen end product.

It’s 1992, and this group of mutants seem scarcely to have aged since their debut in the rebooted “X-Men: First Class” (2011), which was set during the Cuban Missile Crisis. The bald, mind-reading Prof. Charles Xavier (James McAvoy) still runs his school for mutants, while also getting on the government’s good side by sending his adult mutants out on life-saving missions.

The blue-skinned shape-shifter Raven, aka Mystique (Jennifer Lawrence), is in charge of the mission squad, and she warns Prof. X against risking lives unnecessarily to appease the non-mutants running the White House. This argument becomes prescient when, while rescuing astronauts on the space shuttle, the telekinetic Jean Grey (Sophie Turner) almost dies from absorbing a mysterious energy force.

That force enhances Jean’s powers, to the point where she loses control of her abilities. The memory barriers Prof. X installed in her brain as a child collapse, she learns uncomfortable truths about her parents, and she gets angry. Mutants get hurt, other mutants — like the exiled Magneto (Michael Fassbender) — want revenge, and Prof. X and Jean’s boyfriend Scott Summers, aka Cyclops (Tye Sheridan), are in a race to find Jean before Magneto or the authorities do.

Also seeking Jean is a group of aliens in human form, who know the source of that strange energy and want it for themselves. Their leader, Vuk, played by Jessica Chastain as if she had no pigment in her skin or hair, goes Palpatine on Jean, trying to talk her over to the dark side.

First-time director Simon Kinberg knows the X-Men franchise well — this is the fourth in the series he’s written — and he has a good handle on what mutant powers keep the audience involved. He includes plenty of scenes of the teleporting Nightcrawler (Kodi Smit-McPhee) and the weather-controlling Storm (Alexandra Shipp), and the brainy Hank (Nicholas Hoult) makes a few transformations into the blue-haired Beast.

But Kinberg’s handling of action sequences are incoherent, with long stretches of objects and characters zooming about for no apparent reason. The death of a major character is handled clumsily, and it seems less to propel the story than to solve a contractual problem with an actor whose fame long ago outgrew the franchise.

Other plot points feel cribbed from that other Marvel franchise, particularly a rift between mutants that echoes “Captain America: Civil War” and an ending that copies the visuals of “Captain Marvel.” And at the movie’s center, Jean Grey is surprisingly hollow, and that’s on Kinberg, since Turner has shown on “Game of Thrones” that she can be as good as her material.

The “Dark Phoenix” storyline was tried in 2006, in Brett Ratner’s “X-Men: The Last Stand,” which is when the series started to turn sour. Bryan Singer’s “X-Men” (2000) and “X2” (2003), which melded tight action and a thoughtful allegory for xenophobia, showed years before the Marvel Cinematic Universe what a superhero series could do.

Then the franchise splintered. Hugh Jackman’s Wolverine appeared in three solo movies, while a spinoff character, Deadpool, used the X-Men as a comic foil representing stuffy authority among superheroes. Meanwhile, the reboot, starting with “X-Men: First Class,” ran our now-younger heroes through nostalgic period pieces. The series brought back Singer for the time-twisting “X-Men: Days of Future Past” and the execrable “X-Men: Apocalypse,” before accusations of sexual misconduct surfaced and bad on-set behavior got him fired from “Bohemian Rhapsody.”

It’s telling that “Dark Phoenix” doesn’t have the “X-Men” name in the title, following the pattern of the “Deadpool” movies, “The Wolverine” and “Logan,” and the much-delayed “The New Mutants.” (Mark your calendars, in pencil, for April 3, 2020.) Some fans hope, with Fox being consumed by Disney, that the X-Men might hook up with the Avengers in a monster-sized superhero mash-up. Based on the evidence of this 12th movie, it’s time to give the mutants a rest.

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★★

‘Dark Phoenix’

Marvel’s mutants fizzle out in an incoherent action movie that should mark the end of two decades of “X-Men” machinations.

  • Where • Theaters everywhere
  • When • Opens Friday, June 7
  • Rated • PG-13 for intense sequences of sci-fi violence and action including some gunplay, disturbing images, and brief strong language.
  • Running time • 113 minutes